A fiber Bragg grating is a refractive index modulation having a periodic profile, photoinduced in an optical fiber. Writing a FBG comprises two aspects: phase and amplitude. The phase gives the position of the FBG index fringes relative to the assumed underlying uniform pitch, and the amplitude is the magnitude of the index modulation at any given location in the FBG. Variation of the FBG amplitude is often called apodization, since the ends of the FBG must be softened (apodized, or gradually reduced to zero) in order to avoid undesirable group delay and reflectivity ripples, which would result from an abrupt transition from a non-zero amplitude to zero.
To mathematically describe the FBG, the modulation of the effective refractive index of a single mode fiber can be written as:n(x)=neff(x)+Δn(x)cos(kg0x+φg(x))=neff(x)+Re{Δn(x)exp[i(kg0x+φg(x))]}  (1)where the FBG central k-vector is kg0=2π/Λg and Λg is the central period (or pitch) of the grating in the fiber. This index modulation causes a Bragg reflection band at a desired central wavelength λB, which is given by λB=2neffΛg, and where neff is the effective average (i.e. excluding the rapidly varying index modulation) mode index of the single mode fiber. neff can be slowly varying and thus it is written in Eq. (1) as neff(x). Δn(x) is the amplitude of the index modulation, and therefore represents the apodization profile. φg(x) is the residual phase, representing the fringe position relative to the uniform period Λg, 2π of phase corresponding to a fringe position shift equal to the full fringe period Λg. Finally, x is the position along the fiber. Note that the phase information φg(x) can be used to ‘chirp’ the grating, which can be used for dispersion compensation, and also the phase can be used for ‘sampling’, which produces multiple reflection bands that for example may be matched to the standard ITU grid frequencies used in commercial WDM systems. Phase sampling has been described in detail in U.S. patent application Ser. No. 09/757,386, entitled “EFFICIENT SAMPLED BRAGG GRATINGS FOR WDM APPLICATIONS”, which is incorporated herein by reference.
Several methods have been developed for FBG writing using the side illumination of the optical fiber through a phase mask, as illustrated in FIG. 1A (PRIOR ART). Such a typical system includes a source of actinic radiation, for example UV laser 100, projecting light along the optical fiber 102 through a phase mask 104. A scanning mechanism 106 may be used for example to illuminate a long section of fiber 102 through the mask 104, typically with a beam of small diameter (a few mm or smaller). Alternatively, one can scan the mask/fiber pair and keep the writing beam fixed, or avoid scanning entirely and use a writing beam large enough to expose the entire section of fiber required. The mask has a periodic structure of grating corrugations 108 on the surface closest to the fiber 102, which, when illuminated by the writing laser, generates diffracted orders forming an intensity fringe pattern that photoinduces a refractive index modulation along the fiber 102, defining the FBG 110.
The height of the grating corrugations at the mask surface, with peaks and valleys of ±d, has a periodic distribution and can be written ash(x)=d sin(km0x+θm(x))  (2)where the mask has an underlying period Λm and k-vector km0≡2π/Λm, and the residual phase of the mask corrugation function is θm(x). Although we assume a sinusoid here for simplicity, typically the corrugation of the mask will be closer to a square wave, but the exact shape does not affect the general concept represented here. The grating corrugations in the mask cause the writing beam to diffract into multiple orders. The corrugation depth 2d is chosen such that the ±1st orders are maximized and the 0th order is minimized. Typically this depth will be near the size of the UV wavelength (e.g. 2d is about 250 nm). For a uniform mask of period Λm, the two 1st order beams will interfere to produce an intensity pattern with a fringe period (and thus the period of the grating in the fiber) Λg=Λm/2. It is also known in the art to use a mask which is patterned with a non-uniform period (chirp) or phase to produce a similarly varying chirp or phase in the FBG. Thus, the phase information is usually embodied into the periodic distribution of the grating corrugations. The amplitude, and therefore apodization, information, however, is usually introduced in the writing process itself. The simplest method is simply to vary the laser beam power during writing. However, this method causes a variation in neff(x), which leads to severe distortion of the FBG reflection spectrum. To correct this a second pass of fiber exposure is used to equalize neff(x) over the FBG length, but this is more complex and is subject to various uniformity and alignment issues.
A standard technique to achieve apodization without variation of laser power is by controlled wiggling of the mask during writing, such as for example shown in U.S. Pat. No. 6,072,926 (COLE et al). This is illustrated in FIG. 1A (PRIOR ART) where mask wiggler 112 is shown. If the mask is wiggled by a distance more than one fringe period, the fringes can wash out completely. By changing the wiggling amplitude one can control the net fringe amplitude and thus control the index modulation amplitude Δn(x). This method is still mechanically complex and is subject to the variations of the mechanical wiggling system. Moreover, as a result of nonlinear writing sensitivity, this method can still have the undesired effect of varying the effective average index neff(x), distorting the FBG spectrum. In the absence of such a wiggling apparatus, the mask and the fiber can be mechanically joined by a very simple jig (perhaps just a spacer between the fiber and mask and. a clamp to hold them together). This type of mechanical arrangement is likely to have the best thermal and mechanical stability, which can greatly improve the quality of the written FBGs.
Ideally, the apodization information should be included in the mask itself, so that the writing process would simply consist of scanning the mask-fiber with the writing laser beam, without additional mechanical variations, or a simple exposure by a stationary large beam.
A few methods have been proposed in the prior art to incorporate the amplitude information into the mask. One approach uses modulation of the duty cycle (i.e. width), or etch depth, of the grating corrugation on the mask to modulate the intensity of the ±1st and 0th order diffracted beams, such that the visibility of the fringes in the transmitted light is varied. This approach suffers from a number of practical difficulties in achieving the desired flexibility and accuracy of the amplitude profile, and since the visibility of the fringes is modulated, it is possible that it could generate some undesirable variation of the effective average index of the fiber core, neff(x). A summary of these prior art methods can be found in the book by R. Kashyap, “Fiber Bragg Gratings”, Academic Press, 1999 (chapter 5).
Also known in the art is to use interference between two FBG fringe patterns to control fringe amplitude, as disclosed in U.S. Pat. No. 6,307,679 (KASHYAP). However, the two component FBG patterns are written sequentially. As a result, this method has the drawback that the longitudinal position of the fiber must remain very precisely controlled, generally on the scale of 1 nm, between the sequential writing passes of the two FBG patterns. In addition, the writing laser power and beam position and angle must be very precisely maintained between the two writing passes. A number of approaches were also recently proposed in U.S. patent application Ser. No. 10/154,505 by Popelek et al, filed on May 24, 2002 and entitled “Embodying Amplitude Information into Phase Masks”, which use a single illumination of the combination of multiple patterns on the same mask to achieve the required apodization.
In view of all of the above, there is a need for an improved phase mask and a FBG writing method overcoming the drawbacks of the prior art methods discussed herein.